1. Field of the Invention
The present invention relates to a method for manufacturing expandable styrene type polymer particles uniform in particle size by adding a styrene type monomer to styrene type polymer seed particles uniform in particle size and suspended in an aqueous dispersing medium to thus cause the styrene type monomer to polymerize in said seed particles, and impregnating the resulting polymer particles with an easily volatile blowing agent. It provides expandable styrene type polymer particles which are not only excellent in moldability, but give foamed articles high in strength and fine in external appearance.
2. Description of the Prior Art
Expandable styrene type polymer particles release an easily volatile blowing agent (hereinafter referred to "blowing agent") contained therein to readily lose its expanding capability, and the time required therefor becomes shorter with particles smaller in size. Hence, when a mixture of particles different in size is expanded, a difference in expansion ratio results according to the particle size and the strength of foamed articles identical in density is lowered with increasing proportion of large-sized particles. When large-sized pre-expanded particles are included, it gives rise to such problems as deterioration of the charging property into a mold and difficulty to obtain a good foamed article. Furthermore, a foamed article obtained from particles broad in particle size distribution is different in the size of expanded particles, hence, not favorable in external appearance. From such a viewpoint, it is preferred to use those narrow in particle size as expandable styrene type polymer particles for producing a foamed article.
For obtaining expandable styrene type polymer particles, there have hitherto been known methods roughly classified into the following categories;
(1) A method for obtaining particles of a given size by suspension polymerization of a polymerizable monomer, then impregnating the resulting polymer with a blowing agent and finally sifting it.
(2) A method for suspension polymerizing a polymerizable monomer, then sifting the resulting polymer and finally impregnating only the particles of a given range of size with a blowing agent.
(3) A method for pelletizing a polymer to obtain pellets of a given size and then impregnating them with a blowing agent.
(4) The so-called seed polymerization method in which a polymerizable monomer is suspension polymerized, then only particles of a given size are taken out and suspended in water, subsequently a polymerizable monomer is added either continuously or intermittently for further polymerization in seeds, then the grown particles are impregnated with a blowing agent.
As to the method (1), however, all particles are impregnated with the blowing agent, and hence, the particles containing the blowing agent but not suited for intended uses because of their particle size being outside the intended range have to be treated for some other use, which becomes a heavier burden with increasing the amount of production.
As to the method (2), since the polymer particles are used after sifting, the process of producing polymer particles and that of impregnating them with the blowing agent are necessarily separated, which results in complication of the process and also an increase in cost. Further, by this method, too, like the method (1), the burden of treating the particles off the desired range of the particle size for some other suitable use cannot be avoided, either.
In the method (3), too, the pelletizing step is required in addition to the polymer producing step and the step of impregnation with the blowing agent, the overall process being thus highly complicated. The cells formed by expanding the expandable particles are extremely small and it is difficult to obtain a good foamed article.
In order to eliminate the aforementioned defects there has been proposed the method (4) in Japanese Examined Patent Publication No. 49-2994, i.e., the seed polymerization method. According to this method, it is possible to manufacture styrene type polymer particles having the desired narrow particle size distribution according to the uniformity in size of the seed particles suspended in the medium by suspending polymer particles uniform in particle size in water and adding monomers containing a polymerization initiator.
Expandable styrene type polymer particles can be obtained by impregnating in an aqueous suspension the styrene type polymer particles so obtained with a blowing agent, i.e., an easily volatile aliphatic hydrocarbon such as n-pentane, which slightly swells the polymer particles, or by impregnating in an aqueous suspension a blowing agent such as butane and propane which are gaseous at the normal temperature together with a small amount of solvent such as toluene and cyclohexane capable of dissolving the polymer particles.
The expandable styrene type polymer particles so prepared are used as a material for manufacture of styrene type foamed articles. For industrial manufacture of foamed articles, the most common method is to first expand by steam heating or the like the expandable styrene type polymer particles to prepare pre-expanded particles, then charge the pre-expanded particles into a closed mold in the desired shape having many small holes in its wall, heat the pre-expanded particles to a temperature higher than the softening point of a polymer of the pre-expanded particles by letting a heating medium such as steam out through the small holes in the mold to thereby have the expanded particles fused together and then remove a foamed article from the mold after cooling.
The foamed article obtained by the aforementioned method is formed through further expansion of the pre-expanded particles inside the mold to fill the gaps among the particles, but it is extremely difficult to obtain a foamed article completely free of inter-particle gaps. In the vicinity of the wall of the mold, in particular, the charging rate for the pre-expanded particles is lower than elsewhere, hence it is quite difficult to completely fill the inter-particle gaps, and in consequence, in the foamed article obtained there exist inter-particle gaps as superficial hollows. The existence of such inter-particle gaps on the surface of the foamed article not only deteriorates the external appearance of the foamed article, but also causes a decrease in the strength thereof.
For solving this problem, many attempts have been made for improving the expandability by incorporating a plasticizer which has a plasticizing potential to styrene type polymer particles. Incorporation of the plasticizer, however, causes lowering of the softening point of styrene type polymer particles, that of the surface layer thereof, in particular, which causes aggregation of particles in the course of pre-expansion or melting of the surface in the course of expansion molding.